Tundishes are refractory lined metal boxes conventionally used as a reservoir to control the flow of liquid metals between metal working vessels, such as between a ladle and a continuous caster mold. In recent years there has been an ongoing trend to practice "clean" steelmaking to produce high quality steel grades. To this end, efforts have been made to optimize the design and size of the tundish to control the thermal distribution, the quality, and the flow of steel exiting the tundish and to have expendable refractory linings in the tundish which do not contribute any impurities to the liquid steel.
In addition to the outer casing of the tundish, the steel walls and bottom are conventionally lined with at least one permanent refractory lining, usually refractory brick or castable and an inner expendable refractory lining which must be replaced after every steel pour, or at best after a sequence of up to about 20 steel pours.
In a typical tundish, metal flows into the vessel, fills it to a certain level, and exits through one or more bottom ports sometimes called tundish nozzles. In a typical unpartitioned tundish, the outer rim of the bath inside the tundish is a cooler and stagnant zone because this metal does not mix with the metal entering the tundish. Eventually, when this cooler metal leaves the tundish it can cause either skulling or quality problems downstream. In addition, this type of tundish design creates turbulence particularly when a shroud tube is not used on top of the bath which causes increased heat loss and reoxidation of the liquid steel. To prevent this turbulence, weirs have been installed on either side of the inlet stream. This quiets the bath which decreases the possibility of reoxidation and allows inclusions to float to the top of the bath and to become incorporated into the slag.
One problem associated with use of weirs is that the thermally stagnant zones still exist and some of the incoming metal stream can avoid containment in the tundish and rapidly exit through the bottom ports. These problems are alleviated by use of dams located in the bottom of the tundish in conjunction with the weirs. The dams eliminates a rapid exit from the tundish by forcing the steel stream to move in an upward direction.
Another method of improving the steel flow pattern in tundishes is through the use of tundish baffles in place of the weirs and dams. They combine the effect of dams and weirs by directing steel flow through a series of holes in the baffles. An additional benefit of the baffles are their large surface area onto which nonmetallic inclusions in the steel tend to adhere.
The above tundish linings and flow-control devices have been constructed from magnesite tundish boards which were originally used as tundish liners. These are preformed shapes which often contain resin binders. U.S. Pat. Nos. 4,042,229 and 4,043,543 describe this practice in detail. However, problems arose with use of these boards because of their inadequate strength and corrosion resistance which does not enable them to withstand sequential casts of several hours or more. Another problem with the boards is hydrogen evolution from their resin binder if they are not preheated above 1000.degree. F.
Some facilities have tried to form the dams by using high alumina brick, but installation of brick is expensive and time consuming. Forming weirs with brick would be quite difficult.
Monolithic forming of these flow-control devices is most commonly practiced. Cement bonded 60 to 90% alumina and phosphate-bonded high alumina preformed refractory monoliths are typically utilized in service. Major disadvantages to the use of these flow control shapes are their weight and bulkiness which can result in time-consuming, difficult, and dangerous installations. Oftentimes, the permanent refractory linings in tundishes are in disrepair such that these preformed shapes no longer fit into the intended locations. Another problem with the cast high alumina shapes is that they can be eroded during service and inclusions of the refractory can be trapped in the steel. A further disadvantage is the relatively high cost of these preformed, flow-control shapes.
As to all preformed flow control shapes there is the problem of damage thereto in transit from the point of their manufacture to point of use, the lack of a precise fit in any given tundish shape, and the expense of their installation separate from installation of the expendable tundish lining.
Efforts to utilize expendable linings for tundishes which can be applied by spraying, trowelling, tamping and molding as set forth in Canadian Patent No. 1,119,662 have not been successful. Such linings contain materials which act to contaminate the iron and steel, such as resinous binders and oxidizers such as Fe.sub.2 O.sub.3.